1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a secondary battery having an improved seal structure between electrode tabs acting as terminals of the battery and a case, thereby preventing leakage of electrolyte.
2. Description of the Related Art
As portable wireless electronic apparatuses such as a camcorder, cellular phone or laptop computer becomes lighter and requires high functionality, a high performance secondary battery is regarded as a driving power source. Such secondary battery includes nickel-cadmium battery, nickel-hydrogen battery, nickel-zinc battery, lithium secondary battery and the like. In particular, the lithium secondary battery has a long life span and large capacity.
The lithium secondary battery is classified into a Li metal battery and a Li ion battery both using a liquid electrolyte, and a Li polymer battery using a polymer solid electrolyte, according to the type of electrolyte.
The Li polymer battery is classified into a complete solid type Li polymer battery which does not contain an organic liquid electrolyte at all, and a Li ion polymer battery using a gel type polymer electrolyte containing small amounts of organic liquid electrolytes, according to the type of polymer solid electrolyte.
Thus, leakage of the organic liquid electrolyte is serious in the case of the secondary battery containing a organic liquid electrolyte, e.g. Li ion polymer battery, while the complete solid type Li polymer battery has nothing to do with the leakage problem of organic liquid electrolyte.
FIG. 1 is an exploded perspective view showing an example of a conventional secondary battery.
Referring to FIG. 1, the secondary battery includes a battery body 14 in which a positive electrode (not shown), a negative electrode (not shown) and a separator(not shown) are stacked, and a dielectric package 15 for sealing around the battery body 14. The, the battery body 14 and electrode tabs 17 and 17' acting as electrical paths for inducing current formed in the battery body 14 to the outside are connected by connection tabs 16 and 16' respectively provided on the positive and negative electrodes, and the electrode tabs 17 and 17' are installed to be exposed outward by a predetermined length.
The dielectric package 15 is in the form of a dielectric film obtained by forming a heat sealable material layer on a metal base such as aluminum thin film. The dielectric package 15 seals the battery body 14 as follows. That is, while the positive electrode tabs 17 and 17' are partially exposed to the outside of the dielectric package 15, the battery body 14 is put on the dielectric package 15. Then, the dielectric package 15 is folded in half, and the pressure and heat are applied thereto such that the heat sealable material layers formed along the edges of an upper dielectric package 15a and a lower dielectric package 15b adhere to each other, thereby sealing the battery body 14.
FIG. 2 is a section view showing the sealing state between the dielectric package 15 and the electrode tabs 17 and 17' of the conventional secondary battery shown in FIG. 1.
Referring to FIG. 2, upper and lower surfaces of the electrode tabs 17 and 17' are surrounded by the upper and lower dielectric packages 15a and 15b. However, spaces 19 through which the leakage of the organic liquid electrolyte can be made are uaually formed at both sides of the electrode tabs 17 and 17'. Such spaces at both sides of the electrode tabs disrupt complete sealing, the detailed reason of which are as follows.
First, adhesive force between metals forming the electrode tabs 17 and 17' and the heat sealable material layer of the dielectric package 15 is weak. Second, the fluidity of the heat sealable material layer of the dielectric package 15 is poor. That is, for complete sealing, the heat sealable material layer of the dielectric package 15 should show good fluidity when melted by the pressure and heat applied for sealing so as to be flowed into the sides of the electrode tabs 17 and 17' to fill the spaces. However, due to the poor fluidity of the heat sealable material layer, the spaces formed at the sides of the electrode tabs 17 and 17' are not well filled.
Thus, in the conventional secondary battery, the incomplete sealing near the electrode tabs 17 and 17', between the upper and lower dielectric packages 15a and 15b, leaks the organic liquid electrolyte, thereby deteriorating charging and discharging property and reducing the life span of the battery.